Field of the Invention
The present invention relates to an optical filter, and more particularly to an optical filter having a color selectivity.
Description of the Related Art
Previously, a color selective filter is used to select light in a desired wavelength band from an incident light beam. As the color selective filter, typically, a spectral filter (multilayer film interference filter) using multilayer film interference is known. The multilayer film interference filter is obtained by laminating periodic structures where a refractive index or a layer thickness has been adjusted in accordance with the desired wavelength band. Compared with an absorption spectral filter, spectral characteristics of the multilayer film interference filter does not contain absorption, and accordingly there is an advantage that a loss of light intensity or a heat generation does not occur.
On the other hand, in the multilayer film interference filter, an effective layer thickness changes depending on a ray advancing angle in a medium, and accordingly a reflection wavelength varies depending on an incident angle. Therefore, a harmful effect occurs due to a wavelength shift when a light beam incident with an opening angle with respect to the multilayer film interference filter is dispersed.
In order to reduce an incident angle dependency in a dichroic filter or a dichroic prism, Japanese Patent Laid-open No. 2008-58561 and Japanese Patent Laid-open No. 2007-334318 disclose a method of introducing an M-layer with a middle refractive index instead of a L-layer with a low refractive index and a method of introducing an absorption layer to compensate spectral characteristics at the time of an oblique incidence.
However, in the method disclosed in Japanese Patent Laid-open No. 2008-58561, the change of an optical path due to the oblique incidence cannot be effectively reduced. In the method disclosed in Japanese Patent Laid-open No. 2007-334318, a problem such as reduction of a band width due to the oblique incidence occurs. While the incidence angle dependency can be reduced by using a multilayer film in an alternate structure having an in-plane fine structure, it is difficult to sufficiently reduce a reflection wavelength shift at the time of the low-angle incidence.